The invention relates to a tensioning device for the screen fabric in silk screen printing frames. Various tensioning or clamping devices are known by which the screen fabric is fastened in the screen printing frame.
In a first known group of clamping devices, the silk screen printing fabric to be clamped (called "material" for short) is inserted into the C-shaped groove of a tension bar which is formed as the edge limitation of the screen printing frame, the material being fastened by a rod section or band of metal or plastic. The rod section is pushed laterally into the cross-section of the C-shaped groove (called "groove" for short) and this in such a way that the material clamps in the groove under the action of tensile force. In U.S. Pat. No. 3,962,805 a specifically conical rod section with a nose structure is shown, which is placed into the C-shaped groove together with the silk screen printing fabric to be clamped.
In U.S. Pat. No. 3,144,258 the insertion of a flat band is known, while in U.S. Pat. No. 3,962,805 a round rod is used. In U.S. Pat. No. 3,601,912 a flat section is described, which can be bent axially for adaptation to an open L-shaped groove.
Another group of known prior publications designs the clamping section so that section rods are placed into the groove from above in their entire length. In German Pat. No. 1,761,148 and in U.S. Pat. No. 3,482,343 a flat rod locks in a G-shaped groove. In U.S. Pat. No. 3,553,862, two round rods interlock in a specially formed groove.
In U.S. Pat. No. 3,608,482, an approximately semicircular rod locks by rotation in the manner of an eccentric.
All described arrangements have various disadvantages. With the use of a rod section according to U.S. Pat. Nos. 3,962,805 and 3,144,258 the disadvantage is that tolerances in the dimensions of the groove opening and in the rod section have the effect that with respect to the length of the rod section the clamping differs in firmness. A similar effect results when the fabric does not have the same thickness in the clamping region at various points.
The features relating to the configuration of the rod and groove form placed under protection according to the previously described U.S. patents are intended to minimize this disadvantage. In the case of strip sections the disadvantage resides especially in the fact that the force exerted by the tensioned material has a deforming effect on the strip material and forces the latter out of the groove when a certain traction is exceeded, or at least abolishes the clamping. Until now strip material is inserted only from the cross-section (that is, from the end face of the groove), resulting in difficulties when the space in the groove is constricted too much by the material or respectively the friction becomes too great. In the case of the mentioned clamping devices, the thickness range for the materials to be clamped is very narrowly limited, i.e. for different thickness ranges clamping devices of different dimension are needed. With rotatable rod sections (U.S. Pat. No. 3,908,293) the mentioned difficulties are essentially reduced. But they clamp so tightly that they are difficult to release, and this leads to deformations.